This area comprises the delta of the rivers Terraba and Sierpe and is the most extensive mangrove swamp in the country. The most abundant mangrove species is the red (Rhizophora mangle), although tea (Pelliciera rhizophorae) and black (Avicennia germinans) and (Avicennia bicolor).
In this mangrove swamp there is an intricate network of very beautiful channels and islets, and from a boat it is possible to see a great variety of birds, specially pelicans (Pelecanus occidentalis), olivaceous cormorant (phalacrocórax brasilianus), herons and egrets. It is in the north of the Osa Peninsula, to visit it boats can be hired in Sierpe. This mangrove swamp is included in the Ramsar List of Wetlands of International Importance.